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Chemical crystallography, interations of light and organized media, polycrystalline pattern formation, experimental history of science.

 

Crystalline polyhedra are prototypical material objects; light is ethereal. Yet, there has been a surprising reciprocity between crystals and light. Polarization and photon entanglement are crystal-optical discoveries while light scattering has been unsurpassed in the characterization of crystals. We engage this dialogue through investigations of unusual crystalline materials that have informed crystal growth mechanisms and led to the design of new optical materials. Recently, we have focused on the interactions of light with organized polycrystals that occur in biopathological structures, high-polymers, molecular crystals, and simple salts; pattern formation is one of the great organizing principles common to all of the sciences. We develop polarization imaging methodologies for the study of complex aggregates and are especially interested in defining heterogeneities and anisotropies of chiroptical properties.

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Crystal growth traditionally has been viewed as the addition of small units to a monolith. But chemists from New York University (NYU) and Russia’s St. Petersburg State University have discovered crystals that twist and untwist as they grow. The work could lead to a better understanding of the properties of highpolymers, which are used in clothing, liquid crystal displays and other consumer products. The researchers focused on growing crystals from undercooled melts of hippuric acid—a derivative of the amino acid glycine. As

molecules areadded to the end of fine crystalline needles, stresses build up at the tips of the crystals and result in a helical twist. When the crystals thicken from the opposite end of the growing tip, the process is reversed—the twisting is undone by stresses that build-up. The interplay of twisting and untwisting ultimately fixes the crystalline morphology. “This dynamic has not been observed before and points to a much more active process of crystal growth than we had anticipated,” said Bart Kahr, NYU chemistry professor and one of the coauthors of a paper on the research. Kahr’s work is supported by NSF grant number CHE-0845526. Credit: John Freudenthal and Alexander Shtukenberg, New York University

Philip Guston

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